Controllers for the energization of furnaces, air conditioners or humidity modifying loads usually include output switches that are connected in series with the load and with an alternating current power supply. The controller stores a desired temperature or humidity set-point and measures the ambient condition. When the ambient condition deviates from the set-point the controller closes the switch to provide power to energize the load and move the ambient condition toward the set-point.
Prior to the advent of solid state switches of sufficient power capacity and suitable cost for use in such controllers, they all employed hard contacts, usually electro-magnetic relays or mercury switches, as output devices. While low cost, high capacity solid state switches are now available, their use in such controllers has been limited by their susceptibility to destruction because of excess current flow. The switch is connected in series with the AC power supply and the load, which is usually the relay of a furnace, air conditioner or humidity modifying load. If the load shorts out for some reason unusually high currents flow through the switch. While most hard contact switches can carry unusual overload currents without failing, solid state devices are often permanently damaged by currents appreciably higher than their normal capacity. While it is very uncommon for such a relay coil to short out while in normal operation, they may sometimes be shorted out during the installation or repair of the system.
Protection circuits have been devised for power semiconductors used in applications other than these controllers that sense abnormal currents or other abnormal operating conditions and remove triggering power from the solid state switch. For example, U.S. Pat. No. 4,320,434 entitled "Power Semiconductor Protection Circuit with Fault Detection" discloses a circuit that recognizes a potential failure condition and triggers a low power switch that removes switching current from the power semiconductor and resets both itself and the output switch. These circuits have not been previously emplolyed in controllers of the type described above because they were relatively complicated and thus expensive and they generally required connection to the load beyond those normally available at the controller location.